1. Field of the Invention
The present invention relates to a connection component having an integrated ultrasound sensor, wherein the ultrasound sensor has a layer structure having at least two layers, with an electrode layer and at least one layer of a material having piezoelectric properties.
2. Description of the Background Art
Ultrasound testing methods are known from DE 42 24 035 A1 and DE 42 32 254 A1.
DE 10 2004 038 638 relates to a connection component, in the head region of which there is an ultrasound sensor whose structure is formed as a layer structure.
Nowadays, for example, adhesively bonded sensors comprising PVDF films or adhesively bonded sensors made of ceramic material are employed as ultrasound transducers, which are also referred to as ultrasound sensors. Ultrasound sensors made with PVDF films are highly sensitive and are applied, for example, onto screw heads of connection components formed as screws. Application and fixing on a screw head are generally carried out by means of an adhesive layer. PVDF films have the disadvantage that they are not thermally stable. Beyond a temperature of about 70° C. an ageing process takes place in the PVDF film, which in the extreme case can lead to disintegration of the PVDF film. The basic problem with PVDF films is separation of the film over the operating time, or partial separation of the film material, owing to weakening of the bonding force of the adhesive. If the prestress force or the stress force of a connection component configured in the form of a screw is then measured with the aid of an ultrasound sensor, which is made of PVDF film or piezoceramic, then the partial separations which for example occur owing to high thermal loads below the sensor are not visible. Owing to the partial separations and the unknown local position of the partial separations, a false measurement result is obtained by an ultrasound measurement with such a predamaged ultrasound sensor, since the signal propagation times are vitiated owing to the ultrasound signal path length being lengthened by the partial separations. In the event of full separation of the PVDF film or the piezoceramic, in the extreme case an ultrasound measurement is no longer possible at all. The vitiations which occur can lead to considerable errors which can vitiate a result based on a time of flight measurement of an ultrasound signal, or even make it entirely unusable.
It is furthermore known to use a coupling gel in ultrasound measurements, although this is unsuitable for determining a prestress force in a connection component in the form of a screw, since even very small differences when positioning the test head cause ultrasound coupling input time differences which can be greater than the measurement value itself. The required accuracy classes cannot generally be achieved by ultrasound measurements in which a coupling gel is used.
In a vapour deposition method for producing ultrasound sensors, three steps are generally carried out. First, a piezo layer is sputtered onto the cleaned connection elements, generally formed as screws, on one of the two ends. Instead of screws, other connection elements may also be provided with a piezo layer by means of the vapour deposition method, for example rivets or bolts or the like. In a second coating step, a protective layer is applied which is intended to protect the piezo layer against environmental effects. The protective layer is not, however, categorically necessary. Before a further layer can be vapour deposited, namely a metallization layer for the electrodes of the sensors, the connection components are masked. This is necessary in order to be able to produce the electrodes required for the electrical excitation of the sensors. Only by masking is a functional ultrasound sensor obtained from the above-described two- or three-layered system comprising a piezo layer, optionally a protective layer, and a metallization layer. When suitable materials are used, in particular thermally stable and environmentally resistant piezo materials, application of the protective layer can be obviated so that a two-layered system is obtained instead of the three-layered system.
Currently, masking of the plane sides of the connection components is carried out manually by using self-adhesive polyimide film or stamped magnetic masks, although the latter only adhere to components which have ferromagnetic properties. The time outlay, and the concomitant financial expenditure, for manually performed masking of the connection components are considerable. Also, the masking currently carried out manually always entails the risk that the ends of the connection components, for example screws, may be partially contaminated before the vapour deposition of the electrode, which can lead to bonding problems of the metallization layer. Furthermore, the above-described masking method is restricted to simple structures, for example annular structures. As soon as more complex structures are to be made, the manual masking method reaches its limitations.